Help please boat stability

Discussion in 'Stability' started by Miller90, Sep 24, 2019.

  1. Miller90
    Joined: Sep 2019
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    Miller90 Junior Member

    4 max mostly
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I don't know, I can't really say anything too definitive about it, from those pictures, though it does not seem that full forward, which is the one thing I would look for in an offshore 21 footer, and especially a boat with a high cockpit sole, and high structure, and particularly if there is weight in that structure. Have you spoken to the builder or designer of it, and especially if that wheel-house was an afterthought ? Can you mention the make of it ?
     
  3. Miller90
    Joined: Sep 2019
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    Miller90 Junior Member

    Yeah all good mate thanks heaps for ya time I appreciate it I think I might take the roof section off and see what difference it makes I'm unsure of make off hull it definitely looks professionally made compared to the wheel house that is an after thought buy who I brought it off as a project
     
  4. fallguy
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    fallguy Senior Member

    Very light looking forward. I say trouble on a gutcheck.

    So, very interesting..

    Can you add an example?

    I tried to make sense of the method and am missing something.

    So is a roll period of say 9 oscillations in 3 seconds or (9/3=3) ~3 for a boat with a waterline beam of 2 meters bad? Where does he measure waterline beam? The beam forward is pretty narrow and aft quite a bit wider.

    I am only trying to understand how he would interpret your method.

    I hope you understand he has no baseline..even beam he could get wrong....I think the advice could be exceptional.

    Most people measure their beam on an aluminum boat at the top of the hull wide spot, but that is wrong here I believe....perhaps not far off?
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The boat in question is only a moderate vee, and seems to me over-powered, 150hp would have been enough, that big chunk of machinery would be better placed on a deep vee glass boat of that length, that can maintain a high cruising speed offshore, I'd reckon your boat would be good for 20 knots cruise in average offshore conditions, and not much more.
     
  6. DCockey
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    DCockey Senior Member

    How does the size of the engine affect low speed roll behavior?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    It might affect "roll behaviour" at sea, as in rolls over and stays rolled over, if over-driven down sea and broaches, there ain't a lot of boat in the water aft to keep it running straight. I was simply making a side observation, that the boat has more HP than it can use. As the owner says, the deckhouse is an afterthought, and I don't think the designer would have passed it, if as heavy as he thinks it is.
     
  8. Miller90
    Joined: Sep 2019
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    Miller90 Junior Member

    Just thought I would give you's an update I have cut the hard top off from the bottom Windows and has fixed the problem very stable now like 100x better it was that heavy it took 4 guys to lift it off and it was a struggle at that who ever made it must have planned on going in to battle with it lol
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Judging by that business of 4 guys to lift it off, you may have saved yourself from a capsize at sea. You might have got away with it most of the time, but when you in adverse sea conditions, you want a reserve, and it seems the block of flats might have taken that away. The picture of the boat motoring along the river, did seem to indicate quite a bit of extra weight forward. Just apply extra sunscreen to compensate for the loss of shade. That wheelhouse might have suited a larger boat. Happy fishing !
     
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  10. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    yipes! looks top-heavy. boat turns right, top wants to keep going straight = top goes left.

    Another thing you should worry about going off-shore....what if some high winds kick up, especially coming from the wrong direction. Don't want to be forced to choose between taking beam winds and running for harbor.

    Have you tried Phat-Sacks? Big water ballast bladders wake-board boats use to make bigger wakes.
     
  11. JamesRiver
    Joined: Sep 2020
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    JamesRiver New Member

    I would recommend seeing if you can find a local naval architect and ask them to do some stability analysis and an inclining experiment. They should be able to determine what conditions the vessel is safe to operate in as is and provide recommendations if needed (appears to be) to make it safe for your intended use.
     

  12. Ike
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    Ike Senior Member

    Generally speaking no. However, I can recall over a twenty year period two instances where we did do a stability test. One was a Cabin Cruiser type, and all it really needed was redistribution of weighty stuff the owner had on board (20 cases of soda on the fly bridge and a lot of other heavy stuff on board) The owner simply did not understand the relations ship between where weight was stowed and stability. The other was a problem of dynamic instability. Happened before my time there but I watched all the videos and read all the reports. Simple case of a too narrow deep vee with a cabin and a fly bridge. Was fine standing still but did dangerous stuff at speed. Which sounds a lot like what the OP has. Good thing he took the top off. He can probably make a much lighter top with fiberglass and foam or cover it with canvas and some tubes across the top of the cabin.
     
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